Drug resistance causes treatment failure and death in more than 90% of patients with metastatic cancer. Ovarian cancer (OvCa) is a leading cause of death from cancer in women with a projected 14,270 deaths in US in 2014, and is generally diagnosed in advanced stages when the tumor has metastasized and numerous micrometastases with intrinsic or acquired resistance have formed. We aim to synchronize delivery of therapeutic oligonucleotides (ONs) and photodynamic therapy (PDT) agents to OvCa using nanoparticles (NPs), and thus to provide a combination therapy to overcome drug resistance in OvCa. ONs and PDT can provide mechanistically distinct therapies for resistant OvCa but both have limitations when used as monotherapy. ONs are capable of targeting any specific gene that causes drug resistance; however, their therapeutic activity had been constrained by the poor access to their intracellular sites of action because of non-productive endosomal trapping. PDT is a clinically approved anticancer procedure; however, it suffers from sub-lethal cancer cell killing due to its own resistance mechanisms and a short duration of action. We hypothesize that NP-based co-delivery enables complementation of these two modalities, leading to synergism at two levels. Firstly, when delivered together to endosomes in OvCa cells, laser-activated PDT triggers endosomal escape of ONs to their intracellular action sites. Thereafter, the functional delivery of the ONs can target specific genes and overcome drug resistance in multiple modes. Thus, when apoptosis-promoting Bcl-x splice-switching oligonucleotide (SSO) is used, it can guide PDT to an apoptotic pathway. When MDR1 siRNA is used, it can resensitize OvCa to paclitaxel, the first line chemotherapy drug to OvCa. We have prepared ultra-small OvCa-targeted NPs carrying ONs and the PDT drug Ce6. Preliminary studies indicated that laser activation of Ce6 enables functional delivery of the ONs and causes cytotoxicity to OvCa cells. In this proposal, 4 specific aims are designed to test our hypothesis and realize the goal: 1. Elucidate cellular mechanisms for synergism of ONs and PDT and then optimize the NPs for greater synergistic action. 2. Evaluate therapeutic activity of the NPs in 3-D OvCa models that recapitulate some key features of in vivo microenvironment. 3. Test photochemical delivery of Bcl-x SSO in vivo. 4. Test photochemical delivery of MDR1 siRNA. Studies proposed in this grant will thus provide distinct approaches to address the two underlying causes of drug resistance in OvCa: increased intrinsic survival ability and reduced intracellular drug concentration of OvCa cells.